The present embodiments relate to a method and a control sequence determination facility for determining a magnetic resonance system activation sequence.
In a magnetic resonance system, a body to be examined may be exposed with the aid of a basic magnetic field system to a relatively large basic magnetic field of 3 or 7 Tesla, for example. A gradient system is also used to apply a magnetic field gradient. High-frequency excitation signals (HF signals) are emitted via a high-frequency transmit system using suitable antenna facilities to provide that nuclear spins of certain atoms excited in a resonant manner by the high-frequency field are flipped through a defined flip angle in relation to the magnetic field lines of the basic magnetic field. This high-frequency excitation or the resulting flip angle distribution is also referred to in the following as nuclear magnetization or magnetization for short. As the nuclear spins relax, high-frequency signals (e.g., magnetic resonance signals) are emitted, received using suitable receive antennas and further processed. Raw data thus acquired may be used to reconstruct desired image data. The high-frequency signals for nuclear spin magnetization may be emitted using a whole-body coil or a body coil. A structure for this is a birdcage antenna that includes a plurality of transmit rods that are disposed around a patient space of the tomography system where the patient is examined, parallel to the longitudinal axis.
Whole-body antennas may be operated in a homogeneous mode (e.g., a CP mode). A single temporal HF signal is output to all the components of the transmit antenna (e.g., all the transmit rods of a birdcage antenna). In this process, the pulses are optionally sent to the individual components with a phase offset with a displacement matched to the geometry of the transmit coil. For example, in the case of a birdcage antenna with 16 rods, the rods are each activated with the same HF signal offset by 22.5° phase displacement.
With more recent magnetic resonance systems, individual HF signals adjusted for imaging may be applied to the individual transmit channels that are assigned, for example, to the individual rods of the birdcage antenna. A multichannel pulse train that includes a plurality of individual high-frequency pulse trains that may be emitted in a parallel manner by way of the different independent high-frequency transmit channels, is emitted. The multichannel pulse train (e.g., a pTX pulse due to the parallel emission of the individual pulses) may be used as an excitation, refocusing and/or inversion pulse. In this process, homogenous excitation may be replaced with, for example, any manner of excitation in the measurement chamber and also in the patient.
The multichannel pulse trains may be generated beforehand for a certain planned measurement. The individual HF pulse trains (e.g., the HF trajectories) for the individual transmit channels are determined over time as a function of a k-space gradient trajectory using an HF pulse optimization method. The transmit k-space gradient trajectory (e.g., a k-space gradient trajectory or gradient trajectory) is locations in the k-space approached by setting the individual gradients at certain times. The k-space is the spatial frequency space, and the gradient trajectory in the k-space describes a path, on which the k-space is traveled over time when an HF pulse or the parallel pulses are emitted by corresponding switching of the gradient pulses. Setting the gradient trajectory in the k-space (e.g., setting the appropriate gradient trajectory applied parallel to the multichannel pulse train) makes it possible to determine spatial frequencies, at which certain HF energies are deposited. When defining a gradient trajectory, it is provided that relevant regions in the k-space are also traveled. For example, if a region that is clearly defined in the spatial space (e.g., a rectangle or oval) is to be excited, the k-space is also effectively covered in an outer limit region. If, in contrast, only a less defined limit is required, coverage in the central k-space region is adequate.
The user also predetermines a target magnetization (e.g., a desired flip angle distribution) to plan the HF pulse sequence.
A suitable optimization program is used to calculate the appropriate HF pulse sequence for the individual channels so that the target magnetization is achieved. One method for designing the multichannel pulse trains in parallel excitation methods is described, for example, in W. Grishom et al., “Spatial Domain Method for the Design of RF Pulses in Multicoil Parallel Excitation,” Mag. Res. Med. 56, 620-629, 2006.
For a certain measurement, the different multichannel pulse trains to be emitted via the different transmit channels of the transmit facility, the gradient pulse train (with appropriate x-, y- and z-gradient pulses) to be emitted in a manner coordinated thereto, and further control requirements are defined in a measurement protocol. The measurement protocol is created beforehand and retrieved for a certain measurement from a memory, for example, and may, in some instances, be changed locally by the operator. During the measurement, the magnetic resonance system is controlled fully automatically based on this measurement protocol, with the control facility of the magnetic resonance system reading the commands out of the measurement protocol and processing the read out commands.
The “optimum” individual HF pulses of the multichannel pulse trains determined within the optimization method are each complex-value voltage sequences in a time grid of 10 μs and less for each individual independent transmit channel. In practice, the pulse lengths may be, for example, between 2 and 30 ms. Within the optimization methods, which may operate using a Bloch simulation method, these functions may be determined with a high level of quality. The results are numerically stable and mathematically optimal so that target magnetizations with any spatial form may also be generated in the simulations. For individual HF pulses, the individual successive voltage values act in a very non-coherent manner. The curvature behavior of the voltage profile and the phase profile is more like a random function than a constantly differentiable function. This gives rise to the problem that such a function may not be applied with any precisely reproducible quality due to limitations in the transmit hardware (e.g., due to a limited sampling rate). During actual emission of the multichannel pulse trains, target magnetization is not achieved with the required precision (e.g., with respect to a spatial distribution and homogeneity), even though the calculated multichannel pulse trains should supply this quality.